Band pass wave filter



July 16g-1946, w. @ADLEY `4 2,404,270 l BAND PAssfwAvE FILTERy Filed .my 24, 1942 Jus-me45 Z /MPEo/mc:

OUTPUT Patented July 1 6, 1946 BAND PAss WAVE FILTER William E. Bradley Northampton, Pa.,4 assignor to Philco Radio and Television Corporation, Phila. delphia, Pa. a corporation of Delaware .Application July 24,1942-, serial No. 452,229

6 Claims.

This invention relates to` band pass coupling networks or wave filters, and more particularly to a band pass coupling network having a plurality of substantially non-interacting coupling adjustments.

An important class of band pass wave lters frequently employed in the field of communications isthat which utilizes transformers of the double-tuned, over-coupled type. Transformers of this character are commonly employed in the intermediate frequency stages of conventional superheterodyne receivers adapted for use in the reception of amplitude modulated broadcast signals. While these transformer coupled systems have been generally satisfactory in certain limited and conventional applications, including that just referred to, there are other applications in which the transformer coupled system leaves much to be desired. Included in these applications are cascaded wide-band systems wherein it is desired to obtain a particular one of a wide variety of response characteristics simply and reliably. In systems such as these it is difficult, and sometimes impossible, for the: designer to obtain desired response and phase characteristics with conventional over-coupled transformers, and this is especially so where the desired response characteristic is asymmetrical. All of these difficulties are aggravated in production where it is important that all tuning operations be capable of quick, simple, and effective adjustment.

By the present invention a compound coupling system is provided which permits of substantially individual control and adjustment of both the high frequency and low frequency halves of the response characteristic. In over-coupled circuits exhibiting two resonant peaks, the invention permits of individual peak adjustments which are substantially independent of each other, and which can be made as readily as the adjustments of single-tuned circuits. Circuits of this character are especially adapted for use in wide-band systems, such. as the intermediate fre'- quency stages of television and frequency ymodulati-on receivers, and in special. applications which may require response characteristics of an asymmetrical character.

It is a principalobject of this invention to provide a band pass interstage coupling circuit with which a wide variety of response characteristics two-peak response characteristic independently, 'I

and Without any sacrifice in the gain of the stage.

It is a further object of the invention toprovide a Wide-band coupling network capable of giving substantially the maximum gain obtainable in a coupling network between tWo vacuum tubes.

It is another object of the invention to provide y a wide-band coupling network having a novel combination of impedance and admittance coupling.

The invention itself, as well as other objects thereof` will -be understood from the following description and accompanying drawing, in which:

Fig. 1 is a generalized schematic diagram of the coupling system which comprises the present invention; 1

Fig. 2 is a schematic diagram of a specific ernlbodi'ment of the invention; and

Figs. 3 and 4 are explanatory diagrams which illustrate the operation and performance ofthe coupling circuit ofthe invention.

Reference is now made to the generalized schematic of Fig. 1 in which a double-tuned compound-coupled filternetwork is illustrated having an impedance coupling element Zr and an admittance coupling element Yh. The primary circuit of the network Lcomprises an inductance L1 tuned by a tuning condenser C1 and damped by a damping resistor R1. The secondary circuit comprises the inductance L2 tuned by a tuning condenser Cz and damped by a damping resistor Rz. The primary and secondary circuits are preferably isochronous, i. e., tuned to thesame frequency.

While the damping resistors R1 and R2 are shown connected in shunt with the tuning condensers C1 and C2 respectively, it is to be understood that, alternatively, they may be connected t in shunt with the inductances L1 and Lz, or if desired the damping resistors may be eliminated entirely, the required dissipation of energy associatecl with the condensers being achieved, for example, by winding the inductancesv L1 and L2 of a suitable resistance Wire.

two amplifier tubes V1 and V2.

, 3 Y invention that at least one, and preferably both, ofthe coupling devices Yn and Zk be adjustable.

If the admittance coupling element Yu and the impedance coupling element Zk are properly Aadjusted, a symmetrical frequency response char-l acteristic can be obtained which is similar to that provided by a conventional double-tuned overcoupled transformer-system, for. example, similar to thatf'illustratedby the curve (v -a of Fig. '3.

.The novelty of the'present invention resides in the fact that the compound coupling provided by the two coupling elements Yh and Zk permits the. individual peaks of the double-humped response characteristic to be individually shifted in frequency and varied in height or relative response,V4 Yeach rof the adjustments being substantially in-v y dependent of the other, and having substantially no eiectupon the shape of the other peak. If,

for example, the element Y1;` comprises an adjustable inductive admittance, then adjustment of the imaginary part ofthe admittance, i. e., the susceptance, will shift the frequency of the` highcurves-a-b and a-d in Fig. 3. On the other hand if the rea1 part (the conductance) of the admittance Yh is varied, the response or height of the high frequency peak will be affected as is illustrated in Fig. 4 by thecurves e-f, e--y, .and c-h. Adjustment of the imaginary and the real parts, the reactance and resistance respectively,r

of the impedance coupling element Zk will have similar effects upon the shape of the low-frequencyresponse peak.

The invention also contemplates the use of capacitive-coupling elements, and if this change is Reference is now made `to Fig. 2 in whichthere Y,

is illustrated a specific embodiment of the invention employed as a coupling network between In this particular instance the inductive `rform of coupling has been chosen for illustration. In Fig. 2 the adjustable inductance L11 and the vadjustable resistor-R11 comprise the imaginary and real parts, respectively, of the coupling element Yh of Fi'g..1. Similarly'the adjustable inductance Lk andthe adjustable resistor Rk comprise the imaginary and real parts, respectively, of the coupling element Zk. In the circuit'chosen for illustration, the primary and second tuning capacities, C1 and C2, may be provided in whole, or in part, by the output capacity of the amplifier V1 and the input capacity of the amplier V2, respectively.. Plate voltage for the amplier V1 may be supplied from 'a highpotential source B+, through a lter comprising a series resistor R3 and a shunt condenser C3, and thence through the damping resistor R1,

to the anode of V1. In order to isolate the'input grid of the tube V2 from the high voltage applied to the plate of V1 a direct current blocking con- With the form of circuit shown in Fig. 2 adjustment of the inductance L11 effects changes in frequency peak as shownffor example, by the the frequency of the high-frequency peak as illustrated in Fig. 3. MoreoverV the height of the high frequency peak may be varied byV adjustment of the shunt damping resistor Rh to produce various response characteristics such as thoseV illustrated in Fig. 4. Adjustments of the induct- 'ance Lk and resistor Rk produce similar changes in the position and height ofthe low frequency peak. Throughthe adjustmentgofthe elements which Ycomprise the impedance coupling and adlmittance coupling members, an extremely wide .S-.variety of response characteristics may be seacured, anda desired response characteristic may readily bev duplicated in production by means of tuning adjustments which are individually as simpleas those` normally made in single-tuned circuits of the simplest character.

In coupling systems designed to have a symmetrical response characteristic, i. e., in which the heights of the two resonant peaks are equal, the real components of Yh andZk, the elements Rh and Rk respectively, may be omitted.v In such applications only the input damping resistor VR1 and the output dampingy resistor R2 need be employed. In this case the circuit should be designed so that the ratio of R1 to R2 is equal to the ratio of C2 to C1. Thisarrangement will keep the peak heights substantially equal,even when C1 and C2 are unequal, for all settings of `L11 and Lk.l

In the schematic diagram of Fig; 2 adjustable .damping resistors have been associated vwith both Lh and Lk. This has been done for the sake of generality,and suchan arrangement may be used in practice where it is necessary frequently to vary the relative heights of the'two response peaks. In

most practical applications, however, it is preferred to employ only one of the adjustable damping resistors, and then only in conjunction with a predetermined one of the vtwo coupling elements to produce the required asymmetry. V

Although the invention is not limited toany particular method of tuning the inductances Ln and Lk, it is preferred that this be accomplished ductances Lk and'Lk may be replaced `by adjustable condensers of suitable size. i f

It has' been found that the frequency range of the resonant peak controlled bythe inductance Lk is larger, ordinarily,qthan that controlledby the inductance Lk, becauseV the inductance of L11 in practice is usually larger than the inductance Lk. In consequence the inductance L11 tends to resonate with its distributed capacity, making the apparent inductance of this l coupling element larger than the actual inductance of the coil.

Because of Vthis self resonantl action of Ln it is possible to control the high frequency Vpeakfover a considerable frequency range. "It is not so easy t0 obtain wide control of the' :low frequency peak, because the value of Lk is not` lonly much smaller than rthe other inductances involved, -but its leakage inductance is `relatively large. This situation can be relieved by the Vprovision of some' mutualy inductancezbetween4 L1 and L2 so phased :as to produce the opposite signof inductive coupling to that produced by Lk. Under these circumstances Lk can be increased to a larger value, with the result that its tuning range is greatly extended.

The coupling network of the present invention is especially well suited for use in multi-stage amplifier systems, in which a plurality of individual stages (e. g. five or six), each constructed in accordance with the invention, may be connected in cascade. In such a system it is often desirable to stagger the tuning of the various stages, i. e., to peak the individual circuits at predetermined different frequencies. For example, a large number of stages, each having characteristics similar to that of the curve a-a of Fig. 3, but each with its characteristic displaced in frequency a predetermined amount, may be connected in cascade to provide a wide-band, substantially iiat-top, characteristic having greater overall gain and a moredesirable overall phase characteristic than could be obtainedby means of an equal number of identically-tuned attopped circuits connected in casca/de. In other instances it may be desirable to combine ampliiiers having characteristics similar 'to the one designated e-g in Fig. 4 with amplifiers having characteristics of the type designated e-h. In all of these multistage applications, the feature of the network which makes it possible to adjust independently either peak of the two-peak response is of great importance. This makes possible simplified production adjustment of the stages, the various trimmers being peaked at definite frequencies with a signal generator and output indicator, the correct response characteristic then following automatically.

While the invention has been described with particular reference to a specic embodiment and to a particular mode of operation, it will be understood that ythe kinvention is capable of general application and is adapted to other forms of physical expression and is, therefore, not to be limited to the speciiic disclosure, but only rto the scope of the appended claims I claim:

1. A band pass coupling network comprising: a primary circuit including a primary winding and a primary tuning capacitance; a secondary circuit including a secondary winding and a secondary tuning capacitance; and an impedance coupling element connected in .a circuit which is common to both said windings, said impedance coupling element having an adjustable imaginary component; an admittance coupling element connected between a high potential point on said primary winding and a high potential point on said secondary winding, said admittance coupling element having an adjustable imaginary component; said coupling elements being characterized in that they are non-resonant, and of such magnitude that an over-coupled condition is effected between said primary and secondary circuits, the degree of over-coupling being such that a double-peaked frequency-response characteristic is achieved; one of said coupling elements including a real component, the magnitude of which determines the height of one of said peaks vwithout substantial effect upon the height of the other of said, peaks. K I

2. A band pass coupling network, as claimed in claim l, characterized further in that, in addition to the coupling provided between said primary and secondary circuits, said primary and secondary windings are coupled by means including mutual inductance, said mutual inductance being of such sign as to oppose the coupling effect of said impedance coupling element.

3. A band pass coupling network comprising: a primary circuit including a primary Winding and a primary tuning capacitance; a secondary circuit including a secondary winding and a secondary tuning capacitance; a first variable-inductance coupling coil; means for connecting said primary winding, said primary tuning capacitance and said coupling coil in a closed series circuit; means for connecting said secondary winding, said secondary tuning condenser and saidcoupling coil in a second closed series circuit; and ra second variable-inductance coupling coil connected between predetermined high p0- tential points on said primary and secondary windings; the inductance of said coupling coils Y being of a magnitude such that an over-coupled condition is effected Abetween said primary and secondary circuits, the degree of over-coupling being such that a double-peaked frequency-response characteristic is achieved, the position of said peaks along the frequency axis being substantially independent functions of the adjustment of said coupling coils,

4. A band pass coupling network as claimed in claim 3, characterized in the provision of resistive means in damping relation to at least one of said coupling coils, the resistance of which determines the relative height of said peaks.

5. A band pass coupling network as claimed in claim 3, characterized in that, in addition to the coupling provided between said primary and secondary circuits, said primary and secondary windings are coupled by means including mutual inductance, said mutual inductance being of such sign as to oppose the coupling effect of said iirst variable-inductance coupling coil, and thus to permit the use of a larger-than-normal rst coupling inductance, thereby to increase the frequency range obtainable through adjustment of said coupling coil. 

